Global positioning systems (GPS) have revolutionized many aspects of navigation, surveying, high accuracy timing, weapon guidance, customized electronic marketing techniques, and the like. GPS is essentially a system of earth-orbiting satellites that transmit signals continuously to earth. A receiving device on or near the earth's surface receives those signals, and data contained therein, and calculates the distance from the receiving device to each of (at a minimum) four satellites. With knowledge of the transmission time, receipt time, and orbital path of each of the four satellites, the receiver can calculate its three dimensional location in earth's space.
Early embodiments of GPS restricted the use of high accuracy three dimensional location to military and authorized surveying applications by intentionally introducing error into the navigational signals. While this intentionally introduced error has been removed for over a decade, a GPS receiver still experiences error by way of ionospheric and tropospheric aberrations, ephemeris, natural and artificial interference signals, timing and arithmetic error, etc.
Several techniques have been employed to account for the aforementioned sources of accuracy error, and thus yield more precise location measurements. For example, geometric correlation of GPS satellite vehicle (SV) signals, and vector delay locked loop (VDLL), were used as early accuracy enhancement techniques.
Vector tracking has seen a flurry of activity in the wake of geometric correlation and VDLL. Taking advantage of the spatial correlation between satellites has opened vast frontiers of research. Vector tracking loops are characterized by their exploitation of the geometric correlations between satellite tracking channels. For example, if two satellites are close together in the sky, and a receiver moves towards one, it will also have projected motion towards the other. This is the geometric correlation that is leveraged in a vector tracking loop. Satellite-to-satellite geometric correlation is reflected in the nondiagonal terms of the geometry matrix. In general, these nondiagonal terms are nonzero, resulting in correlation between satellites. If more than one frequency is tracked, vector tracking loops also take advantage of spectral correlations among the different signals from each satellite. Scalar tracking loops ignore these correlations. To date, vector tracking research has focused on obtaining real-time solutions without the benefit of precise base station measurements.
Ignoring receiver clock bias, the errors that affect a GPS receiver are normally on the order of a few meters or so. This level of error is much less than the code length (300 m for GPS coarse acquisition [CA] code) and is slowly changing, making vector tracking a realistic solution for the code loop and for the carrier when using a frequency locked loop. However, a few meters is much larger than the wavelength of the carrier (19 cm for GPS L1), making vector phase tracking a challenging proposition. For pure vector phase tracking to be viable, the errors that affect the phase must be mitigated.
For this reason, many vector tracking techniques use a VDLL with a scalar phase locked loop or a VDLL with a vector frequency locked loop (VFLL). Many VDLL/VFLL methods also use scalar phase locked loops.
One technique is scalar in the delay locked loop (and is not a pure vector phase locked loop (VPLL)), with the carrier loop split into two parts, the high-frequency portion is tracked in vector mode, and the low-frequency portion tracked in scalar mode. Others techniques may obtain vector phase lock by estimating and removing atmospheric errors and any initial biases. The individual channel phase discriminator outputs are transformed, using weighted least squares, into position and atmospheric errors. Each error is individually filtered and back transformed to the individual satellite domain to steer the replica carrier.
Using such an approach, periodic reinitialization of the phase biases must be performed if the satellite clock and position errors become too large. This essentially keeps these methods from being pure vector phase locked. These errors can be obviated for implementations with access to precise orbits (e.g., network access for a real-time application or a postprocessed application). However, severe phase multipath affecting all satellites simultaneously is potentially more detrimental to the VPLL than correctable satellite clock and position errors. Since these methods are scalar in the delay locked loop, they require dual-frequency receivers to estimate and remove ionospheric errors.
Other techniques implemented a VDLL/VPLL method by using a Kalman filter to estimate the replica carrier and code of each SV, using all available data. At the same time, the atmospheric errors and receiver clock terms and their derivatives are also estimated. In some embodiments, the Kalman filter also contains states to estimate the receiver's position and derivatives. Again, this method is not pure vector phase locked, since individual-channel biases are accounted for and removed in the Kalman filter. As before, these methods require dual-frequency receivers to estimate and remove ionospheric errors.
None of the aforementioned VPLL techniques uses differential carrier-phase measurements directly in the vector phase loop. Instead, they rely on a postprocessing integer ambiguity technique to get a highly accurate baseline estimate. While more robust at tracking phase than scalar techniques, these methods still do not reach the full potential of pure vector phase tracking.
Others have attempted to use differential corrections directly in the vector tracking loops, however, prior art corrections are limited to code-phase and carrier-frequency measurements vice carrier-phase measurements. Carrier-phase measurements must be used to obtain an ambiguity-resolved differential carrier-phase quality solution directly in the tracking loop.
However, each of the prior are techniques are ineffective at providing an ambiguity-resolved differential carrier-phase quality solution directly in the tracking loops while simultaneously operating in an environment of high-dynamics, noisy signals and intermittent contact with satellite vehicles. As a result, there exists a need in the art for a method of improving spatial location accuracy in dynamic acceleration environments that is also robust to noise signals and intermittent contact with satellite vehicles.